Ok, in C2 projects generally 2 kinds of sounds are used.
Endless background looping music so no sync problems ;-p
Very short little sound effects so with very short delay problems ;-p
In some browsers it's possible to preload sounds to play them quickly when needed, in others only one sound can be played at a time (avoid those)
You could use several looping musics and put some in "muted" state ... "unmuted" then to play again immediatly but of course not always from the start. It's also possible to control sound level for each sound to create a Fade in/Fade out effect so perhaps changing music "smoothly" depending on actions.
You have no way in C2 to have a sound feedback to know really if a sound is audible, you can just control the sound level of each sound.